1. Field of the Invention
The invention relates to an optic fiber hydrophone and applies to the making of multiple-sensor, towed antennas associating a series of hydrophones.
The elasto-optical effect in silica fibers has been recognized as an attractive process for making efficient hydrophones: an isotropic pressure applied to a monomode fiber modifies its length as well as its refractive indices, causing a variation in the phase of the wave which is propagated therein. This phase shift is measured in an interferometrical assembly, generally a MACH-ZEHNDER infererometer which compares the phase shift created in a reference coil, not subjected to the field of pressure, with that created in a detection coil sensitive to this field.
Although the effect to be measured is intrinsically weak, high levels of sensitivity are possible because of low losses in the silica fibers which, through the use of great fiber lengths, enable the cumulation of the elementary phase shifts.
2. Description of the Prior Art
This approach, which uses conventional monomode fibers, comes up against the problem of stray thermal dependence. For, the refractive index of silica varies by 10.sup.-5 per degree Celsius and produces phase shifts during temperature variations. These phase shifts are far greater than the phase shifts created by the pressure variations.
To resolve this problem it has been proposed to use a side-hole fiber (herein also called "FASE"), which improves the pressure/temperature efficiency: instead of a solid cylindrical structure for a standard optic fiber, this fiber has two holes, one on each side of the core which guides the light. This structure can be used, firstly, to create a high level of residual birefringence, in the absence of external stress, obtained during fiber drawing and, secondly, to amplify, anisotropically, the effect of an external stress on the core of the fiber, with a homogeneous pressure applied to a fiber of this type being preferably transmitted along the axis perpendicular to the axes of the holes.
The phase shift in a side-hole structure of this type is measured by comparing the two waves being propagated in the same fiber along the two perpendicular axes. This limits to the problems of thermal dependence: the input-separating function is fulfilled by a coupling at 45.degree., the neutral axes of the side-hole fiber, and the output separating/combining function is fulfilled by an output polarizer/analyzer which is also at 45.degree. to the neutral axes of the fiber.
The use of an interferometer using a fiber of this type removes the problem of the thermal balance of the two arms of the interferometer when they are physically distinct, since the phase shifting due to a temperature variation is the same along both axes.
The interferometer meter has a sinusoidal response as a function of the phase shift between the two arms. For the extreme ends of the sinusoidal curve, there is thus a problem of sensitivity which is conventionally resolved by superimposing a phase modulation on the phase shift created by the sound wave. To make optic fiber hydrophones wherein the measurement is done at a certain distance from the pressure sensor, the need for this modulation then leads:
either to the use of great lengths of connecting fibers at input and at output and also to a great length of electrical cable to supply the modulator close to the sensor;
or to remove the detection coil of the sensor further away from the source and the detector, but then the connecting fibers are an integral part of the interferometer and can produce unwanted phase shifts.
To resolve this problem it has been proposed to use interferometry with white light, namely:
to use a wide spectrum source, hence a source which has no temporal coherence, associated with a first interferometer which has two non-balanced arms, such that the working difference between the two arms is greater than the length of coherence of the source;
then, after passage through a connecting fiber, to use a second interferometer, with the same length of offset as the first one, in order to put the wave trains in coincidence and produce the interferences.
This method enables the creation of interferences as a function of the difference in phase shifts, which are unaffected by the phase shift introduced by the connecting fiber. Furthermore, the insertion of a modulator in one of the two interferometers gives the advantage of sensitivity in the region of the extreme values, for the above-described processing operation.